Sunday, 23 February 2014

Last month, I looked at the three species of marmoset inhabiting the northern half of the Atlantic Forest along the east coast of Brazil. Further south, the forests naturally become cooler - which is to say, that they're merely subtropical, rather than fully tropical. All of the marmosets inhabiting the Atlantic Forest are closely related, belonging to the genus Callithrix, and many of them are capable of interbreeding with one another. They all descend, most likely, from a single species that lived around the early Pleistocene, just before the Ice Ages, and which presumably crossed over what is now the relatively sparsely forested cerrado from the more fertile Amazon to the north and west.

Leaving behind Wied's marmoset, we now hop over the Jequitinhonha River to its southern bank, and a stretch of forest that heads about another two hundred miles or so down the coast. This is the home of Geoffroy's marmoset (Callithrix geoffroyi), also called the "white-headed marmoset". In fact, Geoffroy has quite a few species named after him, and is a key figure in the development of mammalian classification. Living in France around the turn of the nineteenth century, he was an early proponent of the theory of evolution - although, dying fifteen years before the publication of On the Origin of Species, he had most of the details of how it worked wrong.

Sunday, 16 February 2014

The most famous predators of the North American Ice Ages were surely Smilodon and the dire wolves. But, of course, these were hardly the only carnivorous mammals on the continent at the time. For one thing, it's worth remembering that something like half of the species in North America in the mid Pleistocene are still around today. Even where modern species had yet to evolve, their immediate ancestors often had, and would have looked very similar to modern forms. So Ice Age North America would have had its share of coyotes, bobcats, and cougars, not to mention smaller creatures like badgers. And let's not forget jaguars, which entered South America from the north early on in the epoch, and are still found in parts of Mexico today.

But surely Smilodon was the most fearsome predator of its day? Well, probably... kind of. But it most certainly has a contender for that crown.

Sunday, 9 February 2014

In general, among mammals, males are larger than females. We can see this in our own species: on average, men are taller than women, and, again on average, they also tend to have more muscles and a greater overall body mass. Compared with some other species, though, the difference isn't all that great. Seals are an extreme example, with males often many times larger than females. But, in many cases, the difference is more like that with us: it's a noticeable difference, but not a dramatic one.

The reason for this is generally thought to be male-male competition. Males often have to compete for mates, and the bigger you are, the more likely you are to win that contest. It doesn't even have to be physical; the mere fact that you've been able to 'waste' calories on bulking yourself up proves that you're physically fit, and therefore an attractive mate for females. Even the fact that you've survived long enough to grow to large size is a point in your favour.

How this manifests will depend a lot on the mating system of the species concerned. In species that are monogamous, it's not such a big deal, and it's even less so if females are highly promiscuous. It's most noticeable in polygynous species, where one male monopolises as many females as he can, perhaps defending a harem from all comers. This is what happens in seals, and it's also true, for example in deer. Stags, for instance, not only are quite a bit larger than hinds, but they have huge antlers that not only let them fight off their rivals, but also further demonstrate how many calories they've been able to waste growing something huge just for the heck of it.

Saturday, 1 February 2014

Back in August, at the end of my post on the newly discovered olinguito, I said: "Should we expect more new species of mammal to be found, perhaps even large and interesting ones? You can count on it." I also suggested in that post that remote jungles were exactly the sort of place you'd expect to find one. Well, since I wrote that, we have discovered at least one, and possibly two, new species of large mammal that fit exactly that description.

The first was the kaboman tapir (Tapirus kabomani), which lives in the Amazon jungle, and which was described a couple of months ago. I'll refer you to Darren Naish's blog if you want to know about that, since I have nothing useful to add to what he's already said. There is, so far as I can tell, no dispute or argument about the reality of this tapir as a new species - the first new tapir to be described since 1865. The acceptance of other one, published just last week in the online journal PLoS One, appears to not quite be so clear cut.

The animal in question is a new species of river dolphin, and it, too, was discovered in the Amazon jungle. But, before we look at the details, it's probably best if I start by describing what a river dolphin actually is.

Dolphins, on the whole, live in the sea; this is surely not a great surprise. Yes, they sometimes travel a short way up particularly large rivers, and some of them even do it on purpose. But, generally, the sea is where you expect to find them. Now, there is an exception, and I discussed it back in 2012: it's the tucuxi (Sotalia fluviatilis). This spends its entire life in freshwater, and, as I said at the time, it's the only true dolphin that does this.